In a generally known structure of an organic electroluminescent element (hereinafter referred to as “organic EL element”), a light emitting stack including an anode, a hole transport layer, a light emitting layer, an electron injection layer, and a cathode are stacked on a surface of a substrate which is light transmissive. In such organic EL element, light is produced in the light emitting layer when voltage is applied between the anode and the cathode, and produced light is extracted outside through the electrode and the substrate which are light transmissive.
Generally, light-outcoupling efficiency is about 20 to 30% in an organic EL element, meaning that light which cannot effectively contribute to light emission takes up 70 to 80% of a total amount of the produced light. The reason for this is that light cannot be effectively propagated outside where light is perceived, due to total reflection at an interface between materials with different refractive indices, absorption of light by a material, and the like. Consequently, there is a quite high expectation for improving efficiency in an organic EL element by improving the light-outcoupling efficiency.
There have been a quite large number of attempts to improve the light-outcoupling efficiency. Especially among those attempts, a large number of efforts have been made to increase an amount of light reaching the substrate from an organic layer.
Since the organic layer generally has a refractive index higher than or equal to about 1.7, and glass, which often serves as the substrate, has a refractive index of about 1.5, loss of light due to total reflection (a thin film waveguide mode) at an interface between the organic layer and the glass reaches up to about 50% of the total amount of the produced light. Reducing the loss of light due to total reflection between the organic layer and the substrate will make it possible to greatly improve the light-outcoupling efficiency of the organic EL element.
In order to reduce the loss of light due to total reflection, a method employing principles of dipoles has been developed recently (for example, see S.-Y. Kim et al., “Organic Light-Emitting Diodes with 30% External Quantum Efficiency Based on a Horizontally Oriented Emitter,” Adv. Funct. Mater. 2013, DOI: 10.1002/adfm.201300104, 2013). In this method, light-outcoupling efficiency is improved by orienting dipoles.
However, even in the organic EL element employing methods such as the above disclosed in the literature, it can hardly be said that the light-outcoupling efficiency is sufficiently improved, and thus a structure which can improve the light-outcoupling efficiency even further is in demand.